The present invention relates to a hull structure for a multi-hull ship.
Although conventional multi-hull ships of the catamaran type have several well-known intrinsic advantages, one troublesome drawback, however, is that when in motion vessels of this kind move vertically in a manner which is experienced as being unpleasant to passengers and which generates unfavourable vertically-acting acceleration forces on goods transported by such vessels. Consequently, development of multi-hull vessels has resulted in a hull which has a narrow cross-section at the water line, so that a small lifting force is generated by waves that act on the hull. At the same time, those parts of the hull which are located beneath the surface of the water are joined to those parts of the hull which are located above said surface by means of narrow connections. This results in problems with respect to the equipment required to propel the vessel and also results in a greater need for power. Water-jet propulsion is favourable in the case of high-speed vessels, for instance vessels which are built for speeds of up to 40 knots, although water-jet propulsion units are, of course, only suitable for installation at the water line of the vessel concerned. Catamaran type vessels which have a narrow hull waist at the water line (SWATH=Small Water Area Twin Hull) are therefore, in practice, powered by a propeller drive. A screw propeller, however, places a limitation on the speed at which the vessel can be propelled through the water, since the propeller will erode or cavitate when subjected to high loads. Furthermore, the propeller needs to be driven by a complex and expensive transmission from a drive motor mounted in a hull superstructure. Alternatively, the engine can be mounted in part of the hull that lies beneath the water line, although this would present problems with regard to fitting and maintaining the engine, and also with regard to the supply of air, the discharge of exhaust gases and like features, particularly when the vessel concerned is intended for speeds in the order of 40 knots, in which case gas turbines constitute a realistic alternative. SWATH-vessels have, of course, a low load stability, since the part of the hull which extends above the water line has a relatively small cross-sectional area. Consequently, it is necessary to adjust the buoyancy or floating state of SWATH-type vessels during movement of the vessel through the water with the aid of separate means, such as fins, ballast tanks or the like, which naturally represent complications and a cost increase. The buoyancy or floating stability of the hull will also, of course, present a problem when loading and unloading the vessel.